Conventional disk drives employ a servo system that controls the radial position of an actuator arm relative to the surface of a rotating recording disk. The actuator arm supports a read/write head or transducer above a disk surface and ideally over the center of a selected track on the disk surface. For positioning purposes, the read/write head senses servo information embedded in the disk, which is then used to develop a position error signal. The error signal is then used to adjust the position of the read/write head in a direction to reduce the magnitude of the error for purpose of track following. The servo system is also utilized to move the read/write head from one track to another track.
At one time, disk drives were employed primarily within desktop computers, where the disk drives operated in a static environment within the computer on a desktop or table. The computer was in a stable position and there was little likelihood that disk drive would experience shock loading from impacts as a result of being dropped. Today, an increasing percentage of disk drives are being used in portable electronic devices, including laptop computers, notebook computers, palm-held devices, personal digital assistants, music players and other portable electronic devices. A primary problem associated with contemporary designs of such disk drives concerns shock-induced damage from the impact of a falling portable electronic device colliding with a surface. For example, when a device containing a small disk drive falls from a desk or a user's hand onto a hard surface, the shock pulse magnitude generated can be hundreds or thousands times the acceleration of gravity. Shock induced damage to the disk drive or its components is typically less a problem when the portable electronic device is turned off. When turned off, the actuator arm and head element are securely parked at a position off the surface of the disk or on a crash zone on the disk surface. In many cases, the actuator arm may also be latched to further inhibit movement away from the parked position. Therefore, if the portable electronic device is dropped, there is a substantially reduced likelihood that the disk surface and/or head will be damaged. Conversely, if the disk drive is in operation at the time of the fall, the actuator arm is unlatched and the head is likely positioned over the data portion of the disk surface. As a result, damage can easily occur to the disk surface and/or head element.
External shocks such as these yield at least two undesirable outcomes: physical damage of the disk and/or the head and track misregistration. During operation, a shock of sufficient magnitude will cause the head to impact the rotating disk, thereby damaging the magnetic media film, the disk substrate, and/or the head. Further, the shock event generates linear and radial accelerations that apply a moment to the actuator arm. This moment may exceed the ability of the servo system to maintain the read/write head within the allocated tracking error range required for acceptable data integrity, and the servo system may lose track of the actual position of the head element. This problem is exacerbated by increased track density which reduces the acceptable tracking error range. If a shock occurs during the data writing process, the disk drive is in jeopardy of miswriting the data off track, or worse, writing over previously written data on adjacent or nearby tracks.
Thus, it is often advantageous to ensure that the disk drive heads are in a parked position away from the rotating disks prior to impact or positioned over a designated crash zone. In the case of portable computers, this has been accomplished in the past by adding a micro-electro-mechanical-system (MEMS) accelerometer to the computer so that the free fall condition is sensed and the heads are parked prior to impact. For example, some MEMS accelerometers include an outer ring of material that is fixed to a stationary object, such as the motherboard of a computer. A suspended, movable mass is interconnected via a plurality of arms to an inside surface of the outer ring of material. As the MEMS accelerometer is accelerated, inertia causes the resting suspended mass to move relative to the outer ring thereby loading the plurality of arms that connect the mass to the ring. The arms are doped with a piezo-electric material that creates a voltage difference within the arms when loaded. The amount of voltage difference across each of the arms is measured to ultimately yield the magnitude of acceleration. When a disk drive is at rest, for example, sitting on a table, the acceleration measured by the accelerometer is 1 g (where g=force of gravity: 9.8 m/s2). The suspended mass of the MEMS accelerometer will be acted on by gravity and displaced downwardly from the outer ring causing a reading of 1 g acceleration. When the disk drive is dropped, the mass will move relative to the fixed ring, either in line therewith, causing a 0 g acceleration reading, or moving upwardly therefrom, causing a less than 1 g acceleration reading. Thus, when an acceleration indicates less than, or equal to, a predetermined threshold values for a predetermined amount of time, the disk drive is in a free fall condition. Once it is ascertained that the disk drive is indeed experiencing free fall, the voice coil motor that controls the position of the actuator arm is directed to place the actuator arm into a safe location, i.e., to park the actuator arm. When parked, the read/write head or transducer is located away from the rotating disks or over a crash zone so that should the disk drive impact a surface, the head does not strike the disk surface or is already in contact with the surface at a safe zone.
As an alternative, other MEMS accelerometers include a movable mass with a plurality of fingers emanating therefrom that interact with stationary fingers interconnected to a substrate. When at rest, a uniform gap exists between each pair of moveable and stationary fingers. When the mass of the accelerometer moves with respect to the stationary fingers, the gap between each set of fingers is either increased or decreased. The pairs of fingers function as capacitors, altering the space therebetween which changes the capacitance, which, in turn, is measured to identify the magnitude of the acceleration.
The prior art includes the use of accelerometers to detect free fall. U.S. Pat. No. 5,982,573 to Henze (“Henze”), which is incorporated by reference in its entirety herein, discloses a method of sensing acceleration using a MEMS accelerometer and moving the heads away from the disks before an impact occurs. The accelerometer employed is mounted in and secured to the housing of the disk drive. Thus, after a free fall event is detected, a signal is sent from the accelerometer to a processor to cause a signal to be sent to the voice coil motor to park the actuator arm. In other prior art devices, the accelerometer is positioned outside of the disk drive, such as on the motherboard of a computer. In these instances, the command to park the actuator arm must pass through the ATA interface, or similar interface, of the disk drive, and the disk drive must hold the current operation to respond to the command. In each instance, the interface, command, and response time and overhead involved slow or delay any action taken in response to the generated signal. This time lag can be directly correlated to lost reaction time and translates to a minimum drop distance for which corrective action cannot be taken. Conversely, only drops greater than this minimum distance may be detected in time to take corrective action. Unfortunately, even drops less than this minimum distance may produce considerable damage to a disk drive. Moreover, by placing the accelerometer outside of the disk drive, such as on the mother board of a computer, any malfunction of the computer can prevent the signal from the accelerometer from being processed and/or the appropriate corrective signal from reaching the voice coil motor.
Another drawback of the prior art devices and methods for detecting a fall is that they may be fooled into believing that the electronic device is free falling when it is not falling. More specifically, often vibrational loading of the system may be incorrectly identified as a free fall causing an unwanted parking of the head. For example, during travel on a train, airplane, bus or car, or during jogging or dancing, electronic devices are exposed to periodic vibrational accelerations. These vibrations have an extended duration that may cause a detector to falsely conclude a free fall event is occurring and cause the heads to be parked.
In addition to dealing with the effects of vibrations, as well as establishing or tuning the sensitivity of the detection system, inherent errors within an accelerometer must be accounted for. These inherent errors, i.e. “offsets,” result from imperfections in materials, manufacturing and processing of the accelerometer, the manner in which the accelerometer is affixed to the disk drive, variations in environmental conditions, such as temperature, and other factors. The aggregate magnitude of the offset may vary from one accelerometer to the next. However, the threshold value used in the detection scheme should account for the largest possible offsets among a population of accelerometers if the offsets are not identified. For example, the accelerometers used in testing and collecting the data in connection with the present invention have an offset up to 0.25 g per axis, wherein the magnitude of the aggregate acceleration is represented by the equation:
  a  =                    a        x        2            +              a        y        2            +              a        z        2            
These offset numbers should be factored into any threshold used to detect free fall because the magnitude of the offsets is inherent in the accelerometer. For example, with a 0.5 g aggregate acceleration offset, the accelerometer will always record at least 0.5 g even if it is in freefall. Thus, in order to ensure that a fall is detected, the threshold must be set to a value larger than 0.5 g. In a population of accelerometers, some of them may have large offsets, and some of them may have small offsets. Ultimately, the threshold must be artificially increased to account for the worst case offsets; therefore accelerations from vibrations that would not generally be damaging to a disk drive may fall beneath the adjusted threshold value for accelerometers with smaller threshold thereby indicating a fall. If the duration of these accelerations is below the adjusted threshold for a predetermined amount of time, a false indication of free fall will occur.
In order to correctly identify a free fall condition as a predicate, the axis specific offsets must be identified and accounted for in the algorithms that identify the free fall condition. Traditionally this is done by placing a disk drive with interconnected accelerometer on a horizontal hard surface wherein the accelerations in X, Y, and Z directions are measured. In this example, accelerations in the X and Y directions, which are substantially parallel to the measuring table, should be 0, and the acceleration in Z direction should be either plus or minus 1 g, depending on the orientation of the accelerometer. However, often the measured values in each of the three directions are not ideal. For the X and Y directions, the output is the zero gravity offset for these two directions. For the Z direction, the output from the accelerometer is 1 g plus zero gravity offset in the Z direction. The 0 g Z offset is not readily apparent since the orientation of the accelerometer is under an external load of 1 g, not zero gravity. Traditionally, in order to ascertain the 0 g offset in the Z direction, the disk drive must be rotated 90 degrees wherein the X or Y direction is aligned with the acceleration of gravity and the force felt in the Z direction equals 0. The 0 g offset in the Z direction may now be ascertained since, ideally, the amount of acceleration felt in the Z direction should be 0. The drawback of this calibration method is that additional equipment and processes in the assembly line are required to repeatedly rotate the disk drive 90 degrees. Further, additional handling of the disk drive inevitably leads to increased damage.
Thus, it is a long felt need in the field of disk drive protection to provide a method of more accurately detecting free fall so that the head can be parked prior to impact. There is also a need to more quickly determine if a disk drive is in free fall in order to reduce the height from which corrective action may be taken. In addition, a system is needed that allows for innocuous vibrations to be disregarded thereby preventing false indicators of a free fall event. These needs are very dependent on the accuracy of the sensors employed. So it is yet another long felt need to provide a more cost effective method of calibrating the zero-g offset of accelerometers used to detect free fall that also increases their accuracy.